1. Win a Broadcaster or one of 3 Teles! The annual Supporting Member Giveaway is on. To enter Click Here. To see all the prizes and full details Click Here. To view the thread about the giveaway Click Here.

Need some help with Princeton Reverb Build

Discussion in 'Amp Tech Center' started by KBarW, Feb 24, 2020.

  1. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

    Posts:
    2,303
    Joined:
    Dec 30, 2019
    Location:
    california
    Hope we didn't harsh your mellow.

    I think it's great you were able to build the kit. It takes a great deal of patience and skill. My hats off to you.

    I hope I didn't come off as bashing Mojotone. They provide parts and services for musicians that are not always easy to come by. I have heard their tech response time is excellent. It just leaves me shaking my head when a well known kit is sold without remedies to known issues. I hope they use your critique of the instructions to make it better for the next builder.
     
    King Fan likes this.
  2. KBarW

    KBarW TDPRI Member

    Posts:
    97
    Joined:
    Feb 28, 2012
    Location:
    New Braunfels, Texas
    I'm just yanking your chain. You guys have been great and taught me a lot....mostly about how much I don't know. Now, it's up to me to either move forward in this education or not. Time will tell. You never know what you might decide to do tomorrow.

    I'm just looking forward to playing some gigs on my newly-modded, short-necked Tele through my new Princeton and see if it was all worthwhile.

    I'm sure it will be.
     
  3. corliss1

    corliss1 Friend of Leo's Platinum Supporter

    Posts:
    4,455
    Joined:
    Sep 13, 2008
    Location:
    Lansing, MI
    Ooooh I also use a short scale Tele!
     
    KBarW likes this.
  4. KBarW

    KBarW TDPRI Member

    Posts:
    97
    Joined:
    Feb 28, 2012
    Location:
    New Braunfels, Texas
    I'm really liking mine. I haven't really played my Tele in years. I bought it brand new in 1989. It's an American Standard. I've always thought about putting a new neck on it and finally did it. Got one from Warmoth. Maple. I put 10 coats of Tru Oil on it with some steel wool every few coats. Came out nice.
     
  5. dan40

    dan40 Tele-Afflicted

    Posts:
    1,975
    Joined:
    Aug 19, 2015
    Location:
    Richmond Va
    Just keep in mind that while this pair of power tubes may be running somewhat "in spec" with -31v of bias voltage, the next pair that you install in the future may not be. A new pair could bias up too hot and redplate or possibly too cold and sound awful. Fender was able to set the bias voltage at -31v in every Princeton they made back then because the quality of tubes was much higher and most 6v6's would bias up in a safe range. These days it's a gamble with current production 6v6's and you can not be sure if every pair will bias up in the safe range. Also keep in mind that many amps are running at higher voltages throughout because of higher wall voltages these days. This can also cause the tubes to run hotter which would require increasing the bias voltage to accommodate for it.
     
    King Fan and KBarW like this.
  6. KBarW

    KBarW TDPRI Member

    Posts:
    97
    Joined:
    Feb 28, 2012
    Location:
    New Braunfels, Texas
    Covid 19 boredom has officially overridden my desire to not open up the amp again.....

    I think I'm going to put in the bias pot and while I'm at it - the NFB switch and possibly some other little mods.

    As I've been sourcing parts - I came across a bit of a lack of education and/or detail in the Robinette plans. i.e. the NFB switch shows the values of the two resistors at 2.7 and 5.6k BUTTT....it doesn't saw how many watts. I can find several with the correct "k" but with varying wattages.... 1, 2, 3, 5..... How do I know which to buy when it's not specified?

    Also, this seems much less daunting than it did right after I finished the amp.....I guess my brain has forgotten the pain and is willing to dive back in....
     
  7. corliss1

    corliss1 Friend of Leo's Platinum Supporter

    Posts:
    4,455
    Joined:
    Sep 13, 2008
    Location:
    Lansing, MI
    Tiny - those don't handle any wattage.

    I only buy 1W resistors and higher just so I never have to wonder. Original Fender resistors in that location or 1/2W.
     
    AlbertaGriff and King Fan like this.
  8. KBarW

    KBarW TDPRI Member

    Posts:
    97
    Joined:
    Feb 28, 2012
    Location:
    New Braunfels, Texas
    What took you so long? It's been a whole minute since I asked....
     
    AlbertaGriff and King Fan like this.
  9. corliss1

    corliss1 Friend of Leo's Platinum Supporter

    Posts:
    4,455
    Joined:
    Sep 13, 2008
    Location:
    Lansing, MI
    Sorry! I'll try and type faster next time!!!!!!!!!!!
     
    King Fan likes this.
  10. KBarW

    KBarW TDPRI Member

    Posts:
    97
    Joined:
    Feb 28, 2012
    Location:
    New Braunfels, Texas
    Say, does it matter whether I use Ceramic or Metal film resistors?
     
  11. corliss1

    corliss1 Friend of Leo's Platinum Supporter

    Posts:
    4,455
    Joined:
    Sep 13, 2008
    Location:
    Lansing, MI
    Usually ceramics are the bigger 5-10W guys. Lots of debate on resistor types, but metal film is perfectly fine.
     
  12. KBarW

    KBarW TDPRI Member

    Posts:
    97
    Joined:
    Feb 28, 2012
    Location:
    New Braunfels, Texas
    Well, I thought I was getting my head around this....been watching some videos and what I've found it that no one seems to tell you exactly where the testing points are. They talk about plate voltages and dissipation, but never show you where they are putting the test leads to find the numbers. Everyone seems to assume that the viewer already knows where the points are that correspond to their verbiage....
     
  13. KBarW

    KBarW TDPRI Member

    Posts:
    97
    Joined:
    Feb 28, 2012
    Location:
    New Braunfels, Texas
    Thanks, I went ahead and ordered some before everyone closed for the weekend, along with a pot...I'm confident that I can install it, just don't have a clue where I should be testing for the various voltages....

    I mistakenly thought I could just hit a point on the circuit somewhere and that would be my bias reading. I guess that's not the case.....
     
  14. koolaide

    koolaide Tele-Afflicted

    Posts:
    1,002
    Joined:
    Oct 11, 2009
    Location:
    south carolina
    Just checking back in. Got caught up on thread. Glad to see you have a successful conclusion, and are filling in your knowledge gaps. I feel your pain. I am in the process of setting up an Isolation transformer, and learning about oscilloscopes. (Filling some of my many knowledge gaps)
    Oh, and Uncle Doug has a bias video series and he tells where he puts the probes;)(Sorry couldn't resist)

    Again congratulations on your successful build.

    Peace,
    Jim
     
    Kevin Wolfe likes this.
  15. KBarW

    KBarW TDPRI Member

    Posts:
    97
    Joined:
    Feb 28, 2012
    Location:
    New Braunfels, Texas
    Thanks,

    I watched his video on setting bias, but he only showed the meter and never where he took the measurements.... I didn't see any others that addressed it. If you know which one shows the test points, I'd love to get a link.

    I hope to get the bias pot and the NFB switch done next week when the parts come in.
     
  16. koolaide

    koolaide Tele-Afflicted

    Posts:
    1,002
    Joined:
    Oct 11, 2009
    Location:
    south carolina


    Also watch part 1. Be safe. Be sure to drain the caps.

    Here is one where he makes Princeton Reverb adjustable bias.


    Happy Biasing
     
    Last edited: Apr 5, 2020
  17. KBarW

    KBarW TDPRI Member

    Posts:
    97
    Joined:
    Feb 28, 2012
    Location:
    New Braunfels, Texas
    Gonna watch it right now. I've watched the one on putting in the pot on a Princeton....I'll check out the first one right now.

    I know that caps can store a lot of energy so I've always been careful about 'em.....but how exactly to you drain one? I can't find a little spigot anywhere.....
     
    Kevin Wolfe likes this.
  18. corliss1

    corliss1 Friend of Leo's Platinum Supporter

    Posts:
    4,455
    Joined:
    Sep 13, 2008
    Location:
    Lansing, MI
    No spigot in your amp??? Weird...

    I use a 1K 10W or 2K 10W resistor with some clip leads, clipped to one of the B+ nodes. Whatever is easiest to get to. ON that amp...the pin on the rectifier that goes out into the high voltage supply for the rest of the amp might be an easy spot.

    This is AFTER the amp is off, and make sure you remove it before you turn it back on. If not, you'll cook the resistor quickly, ask me how I know...
     
  19. KBarW

    KBarW TDPRI Member

    Posts:
    97
    Joined:
    Feb 28, 2012
    Location:
    New Braunfels, Texas
    Thanks. I looked up some videos on how to drain the caps. Shouldn't be hard to build something.....just don't keep resistors on hand - but, I think I have some from some old crossovers....
     
  20. KBarW

    KBarW TDPRI Member

    Posts:
    97
    Joined:
    Feb 28, 2012
    Location:
    New Braunfels, Texas
    Ok.....while waiting for the parts to show up today to begin installing the bias pot and adding a NFB switch, I put the amp on the bench just to run back through everything.

    I have to tell you that I spent over 20 years in the computer business along with 40 years of setting up and building sound systems .... so I do understand how to troubleshoot a problem.

    However, for some reason that I still can't figure out - the Vibrato started working.

    I've had the footswitch hooked up all along and living in the back of the cab. So once it was on the bench I plugged in a guitar and put the switch through its paces. Reverb switch worked fine. Vibrato switch did nothing - still dead. Unplugged the switch entirely and checked the reverb - still good. Checked the Vibrato....now it's working. WTH???

    Put the footswitch back in - figuring this is where my problem is (even though I know I tested it without the footswitch before). Works fine. Turns on and off like it should. Even though just 5 minutes before - it didn't.

    Only thing I can figure is that the footswitch wasn't all the way in (I'm using one TRS instead of the two RCAs). But, that still doesn't account for it not working without the footswitch in when I first tested it and couldn't get it to work.

    So, I humbly apologize for dragging you into my lack of Tremolo issue. Please proceed to berate, laugh, or throw used strings at me....I deserve it.

    Although, I'm still not sure what was wrong.

    Now....to decide whether to open it up and install the bias pot and the NFD switch.....I already know the consensus of all of you guys. :)
     
    corliss1 likes this.
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.